In piston engines, particularly 4 stroke petrol engines, their efficiency is reduced when the engine load is reduced. At the end of the exhaust stroke a residue of exhaust gas is left in the cylinder. This exhaust residue is hot and causes the temperature of the incoming fuel/air mixture to be increased. A high temperature of the mixed gases at the start of the compression stroke reduces the fuel efficiency of the engine. Conversely, a reduction in the temperature of the gases at the start of compression would increase the efficiency.
The efficiency of a piston engine is also increased when the speed of combustion is increased.
Also the efficiency of a piston engine is increased when the temperature rise during combustion is increased.
Furthermore the efficiency of a piston engine is increased when the combustion pressure or the effective compression ratio is increased.
This invention aims to increase fuel efficiency in all the above respects which may lead to reduced exhaust emissions.